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Dogs wag their tails to the left or right depending on how they're feeling, Italian researchers say.

Tails wag to the right when dogs are happy and see something they want to approach and to the left when they are frightened and confronted with something they want to run away from, say the researchers.

Professor Giorgio Vallortigara of the University of Trieste and team describe these "striking asymmetries in the control of tail movements" as another example of how the right and left halves of the brain control different emotions.

They publish their findings in a recent issue of the journal Current Biology.

The researchers tested 30 pet dogs, 15 males and 15 females with an age range of one to six years.

They placed the dogs in a large rectangular wooden box covered with black plastic to prevent them from seeing out.

Vallortigara and colleagues filmed each dog's response to four different visual stimuli: the dog's owner, an unknown person, a dominant, unfamiliar dog and a cat.

"When faced with their owner, dogs exhibited a striking right-sided bias in the amplitudes of tail wagging," the researchers write.

Tails kept wagging to the right when dogs were shown an unfamiliar person and the cat.

The human stranger elicited less wagging than the owner, and the cat - a four-year-old European male placed in a small metal cage - the least wagging of all.

But when shown a large, unfamiliar and intimidating four-year-old male Belgian shepherd malinois, also kept in a cage, the tails leaned consistently leftwards.

Dogs also wagged their tails to the left when they were on their own, suggesting that they like company.

"This work shows that even a single medial appendage can show lateralisation and so reflect which side of the brain is active at the time. Also, it provides an excellent way of assessing the reactions of dogs to people, other animals and even different environments."

However, it is unlikely that people can benefit from this discovery when approaching a dog.

Dogs tend to move about constantly, making the right-left bias often so subtle it can only be spotted with video analysis.

"Our study provides direct evidence that also in a non-human species the anterior regions of the left and right hemispheres are specialised for approach and withdrawal processes," Vallortigara says.

Previous studies in humans have shown the same dichotomic partition.

While the brain's left hemisphere, which controls the right side of the body, is associated with a sunny disposition, the right hemisphere is associated with negative emotions and retreat.

"The fact that the same partition is found in animals suggests that brain asymmetry is quite ancient," Vallortigara says.